I wrote a simple argument completion skeleton and pushed it to an Eclipselabs repository. It's not as good as Precise, i.e., you need one extra <ctrl+space>, but it does not need any JDT patches. Personally, I think this it's okay but that's something you have to decide. Below is a screenshot showing it in action. The code is here . It also contains two examples how to get all accessible fields, locals, and methods (once using the IJavaElement APIs, the other using the compiler bindings API).
Please check out the example plugin, start it in your Eclipse runtime, and check for the proposal below. Let me know what else you may need to know.